1. Field of the Invention
The invention involves the art of straightening of kinky hair by a lithium relaxer which is made by a novel process.
2. Description of Related Art
Hair relaxer (straightener) compositions presently known to the art are highly alkaline, oil-in-water emulsions which derive their chemical reactivity from either (1) alkali metal hydroxides, (2) quaternary ammonium hydroxides, or (3) guanidinium hydroxide dissolved or suspended in the water phase of these hair-treatment formulations such that the pH values of these emulsions are in the range of from 12 to 14. Among those skilled in the art, it is widely and generally accepted that it is the hydroxide ion, which is the alkaline chemical species common to all three of the above classes, that is the essential active ingredient in these "strong-base" relaxers. In fact, it has be firmly proven that, when inside the cortex of the hair, hydroxide ions readily abstract acidic alpha protons from the cysteine moieties of hair keratin leading to reversible beta elimination of alkyl disulfide (opening of crosslinks) with the concomitant formation of dehydroalanine. It is as these crosslinks continuously open and reform that kinky hair, under mechanical stress, is relaxed to a permanently straight configuration.
Although it is the hydroxide anion which is responsible for initiating the chemical reactions within the hair shaft which lead to straightening, it is the cation with which the hydroxide is associated that has distinguished among the various relaxer types. Prior to 1979, only those one-component ("no-mix") hair relaxer compositions deriving from either sodium or potassium hydroxides were known. In 1979, Carson Products Company (U.S. Pat. No. 4,304,244) introduced the mix type "no-lye" relaxers containing guanidinium hydroxide. Because guanidinium hydroxide is not stable for long periods in aqueous solutions, it must be prepared fresh just before using. Guanidinium hydroxide is generally prepared by mixing an inorganic alkaline earth hydroxide with an aqueous solution of a salt of the strong organic base guanidine, where the anion of this salt is capable of being precipitated by the cation of the alkaline earth hydroxide. In commercially available products of this type, the guanidinium hydroxide is generally prepared using calcium hydroxide and guanidine carbonate.
There are presently two principal defects in all of the prior art hair relaxers. These are as follows: (1) because of their high alkalinity, all can potentially cause scalp irritation and/or injury during the relaxing treatment; and (2) because all are strong-base relaxers capable of dissolving hair keratin, all can overprocess the hair causing irreversible damage to the fibrous keratin structure leading to hair breakage. It has been a continuing goal of researchers in this industry to correct these serious defects.
The scalp irritation potential of chemical relaxers has been reduced somewhat through the formulation of better and better cosmetic emulsions containing high levels of skin-protecting oils such as petrolatum and mineral oil. And, most importantly, it was discovered that relaxers deriving from guanidinium hydroxide are inherently less irritating to the skin and scalp than those deriving from the alkali metal hydroxides. Significantly reducing the irritation potential of hair relaxers, however, remains a major technical challenge to relaxer formulators.
The problem of overprocessing is manifest in the strong-base chemistry of these formulations. With strong bases which are capable of totally dissolving hair, the only control a formulator has in designing safer relaxers is to adjust the concentration of the active ingredient to an optimally effective level. Then, it is up to the user to carefully time the treatment and stop the process when the hair is straight and before it is damaged. Salon professionals learn these skills through training, while consumers of home-use, kit-type relaxers learn through trial and error.
Like sodium and potassium hydroxides, lithium hydroxide is also an alkali metal hydroxide, and indeed in the 1980s lithium hydroxide was used to formulate commercially successful hair relaxers of the "no-mix" type. Interestingly, those skilled in the relaxer art now make no real distinction among sodium, potassium, and lithium hydroxides as is attested to in the following excerpt from a recent relaxer patent specification (Akhtar et al., U.S. Pat. No. 4,950,485, col 7, in 16): "Alternatively, a cosmetic cream base for use directly as a no-base hair relaxing composition can contain a water-soluble alkaline caustic material which is capable of both bringing the pH of the composition to a value of about 12 to about 14, and acting as the sole hair relaxing agent. Alkali metal hydroxides, including sodium hydroxide, potassium hydroxide and lithium hydroxide may be used as the water-soluble alkaline caustic material. Sodium hydroxide is preferred and may be present in amounts from about 1 to about 3 weight percent of the total composition, preferably from about 1.5 to 2.5 weight percent."
Unlike sodium and potassium hydroxides, however, an aqueous solution of lithium hydroxide is reportedly a "weak electrolyte" (lower electrical conductance), which is evidence of incomplete ionization, which in turn defines lithium hydroxide as a classical "weak base'. Commercial hair relaxers made with lithium hydroxide as the active ingredient typically have pH values of around 12.2 to 12.7; whereas, those made with sodium hydroxide or guanidinium hydroxide generally exceed 13.5. This being the case, one can only guess the answers to the following two questions--(1) If indeed, the hydroxide anion is the species solely responsible for the chemical reactivity of hair relaxer formulations, how can a lithium hydroxide relaxer having a hydroxide ion concentration which is lower by a factor of at least 10 be just as effective in straightening hair as a sodium hydroxide or guanidinium hydroxide relaxer? (2) If the pH is significantly lower, why does the incidence of skin and scalp irritation experienced by users of lithium hydroxide relaxers equal or exceed that of sodium hydroxide relaxer users?
In an attempt to answer the two questions posed above, a search of the technical literature was conducted to locate a value for the dissociation constant for lithium hydroxide: ##STR1##
One reference (Darken and Meier) reported K=1.2 based on electrical conducted. Surprisingly, when used to solve Eq. 1, the literature value of K did not predict the pH of lithium hydroxide relaxers (usually about 12.6). Moreover, it did not predict the pH of simple solutions of lithium hydroxide prepared in the laboratory. For example, the aqueous phase of a typical relaxer is about 1 molar in lithium hydroxide. A value for K.sub.x of 1.2 predicts that the pH would be 13.72. When 1 molar lithium hydroxide solutions were prepared in the laboratory, the pH was 13.01. Because the relationship between the pH and the hydroxide concentration of a solution is exponential, a discrepancy in pH of +0.71 of a pH unit represents nearly a 6-fold difference in predicted vs. experimental molar hydroxide concentration.
One explanation for this very large experimental discrepancy is that the value of K=1.2 (based on electrical conductivity) of lithium hydroxide solutions describes something other that the simple dissociation of LiOH into Li.sup.+ and OH.sup.- ions.
Although lithium is classified in the periodic chart as an alkali metal, in many respects it is grossly different from sodium and/or potassium. In fact, in all salt-forming reactions, the "ionic" bonds between lithium and oxygen (and other electron-rich atoms) are so strong that these salt bonds behave to a large extent like covalent bonds rather than ionic bonds. For example, lithium stearate (an "alkali metal soap") is only minimally soluble in water; whereas the sodium and potassium stearates are highly soluble. Lithium carbonate is soluble in water only up to about 1.5 gm per 100 gm of water, while sodium and potassium carbonates are extremely water soluble.
If the Li-O bond of LiOH is more like a covalent bond than an ionic one, then a secondary equilibrium can be established whereby hydroxide ions abstract protons from undissociated LiOH molecules to produce a LiO.sup.- species (lithoate) according to the following equation: ##STR2##
Allowing for the formation of LiO.sup.- according to Eq. 2 and combining Eq. 1 with Eq. 2, the following can describe the dissociation of LiOH. ##STR3##
An assumption can be made that Eq. 3 (not Eq. 1) now describes a solution of lithium hydroxide in water and that K.sub.xy is the dissociation constant determined by Darken and Meier to have a value of 1.2. Based on this assumption, numerical values for K.sub.x and K.sub.y can be calculated to be 0.263 and 17.35, respectively. When these dissociation constants were used to calculate the pH of aqueous solutions that were 1 molar in lithium hydroxide, the equations predicted that the pH was 12.99, which agreed well with the experiment determined value of 13.01. The complete description of all of the soluble species in a 1 molar lithium hydroxide solution is as follows:
OH.sup.- =0.143 molar PA1 LiO.sup.- =0.357 molar PA1 LiOH=0.144 molar PA1 Li.sup.+ =0.499 molar PA1 pH=12.99 PA1 OH.sup.- =0.804 molar PA1 NaOH=0.195 molar PA1 Na.sup.+ =0.804 molar PA1 pH=13.74
The 1 molar lithium hydroxide solution described above should be compared to an analogous 1 molar sodium hydroxide solution whose soluble species are as follows:
Clearly, the concentration of hydroxide ion in a 1 molar lithium hydroxide solution is lower by a factor of more than 5 than the hydroxide concentration of an analogous sodium hydroxide solution. Chemists skilled in the art of hair relaxer formulations would certainly agree that if one made a sodium hydroxide relaxer whose aqueous phase contained only 0.143 molar NaOH (0.3 wt. % in the formula), a relaxer so weak would take many hours to relax hair. This being the case, it is likely that the LiO.sup.- (lithoate) species, a base other than hydroxide, is the principal active ingredient in lithium hydroxide relaxers.
In a number of applications-oriented studies, lithium hydroxide has been observed to behave differently than sodium hydroxide and/or potassium hydroxide. For example, lithium hydroxide is sometimes used in the tanning of hides. It is reported to be absorbed into the skins to a greater extent than other alkalis, but it causes less swelling. Because from a chemical viewpoint, hair keratin and skin keratin are very similar, one might expect lithium hydroxide (possibly as lithoate ion) to penetrate the hair shaft more readily and with less swelling than other alkalis. This being the case, one would predict that lithium hydroxide might be a good hair relaxer even though it has a substantially lower hydroxide ion concentration than other types of alkaline relaxers. Moreover, if lithium hydroxide is absorbed readily into animal hides, it might be expected to be absorbed readily into the human skin and scalp; thus, it might be highly irritating to the skin even thought the pH is lower than with other alkalis.
When lithium hydroxide relaxers are made in a manner and by a process analogous to the preparation of sodium hydroxide relaxers, the skin and scalp irritation potential of the resulting emulsions is very high. The incidence of caustic burning is much greater than for sodium hydroxide relaxers having comparable molar concentrations of hydroxide ion. Skilled formulators, however, have discovered that the addition of several percent of calcium oxide or calcium hydroxide to the lithium hydroxide relaxer formulations somehow attenuates the activity of the active chemical species with regard to skin and scalp irritation without compromising hair straightening efficacy. The precise mechanism by which calcium hydroxide and/or calcium ions change the composition is not known, but we can postulate that calcium hydroxide may cause some sort of shift in the chemical equilibrium causing a change in the concentration of one or more of the active species.
With this improvement in the composition of lithium hydroxide relaxers, irritation is lessened to a measurable degree in most batches, but the production process is not highly reproducible and the results in this regard are unpredictable. Many batches still have a very high potential for skin irritation and others have a much lower potential. It has been generally observed that those lithium hydroxide relaxer batches having a high propensity for skin irritation become somewhat milder and less irritating with aging, but the aging time must be on the order of many months or several years. This suggests that lithium hydroxide (with calcium hydroxide) relaxers made by present formulation processes do not quickly and reproducibly reach a thermodynamically stable state. Whatever the reason for the variability and unpredictability of present lithium hydroxide relaxers, these relaxers have achieved far less commercial success than the sodium hydroxide relaxers and the guanidinium hydroxide relaxers.